The invention relates to a process for the production of an aluminium hydroxide of improved thermal stability which can be incorporated in plastic composite materials whose processing temperatures exceed 200xc2x0 C.
Aluminium hydroxide (Al(OH)3) is widely used as a flame retardant in polymeric systems. On heating to above a temperature of ca. 200xc2x0 C., the aluminium hydroxide begins to decompose to aluminium oxide and water. This reaction is endothermic, thus absorbing heat from the of the polymer. The amount of water released corresponds to 34.6 wt. % of the aluminium hydroxide. Both these factors act to suppress a burning process and reduce the formation of combustible gases by keeping the temperature down.
To be useful as a flame retardant, the aluminium hydroxide must remain stable during processing of the polymer. This renders it unsuitable for polymers which are processed above 200xc2x0 C.
Prior art indicates that one way around this limitation is to preheat the aluminium hydroxide to eliminate a part of the water which is released at elevated temperatures. However, there are inherent disadvantages in this approach:
The amount of water (vapour) available for the flame retardant action is reduced.
A portion of the aluminium (tri)hydroxide on heating above 200xc2x0 C. converts to aluminium oxide hydroxide (AlOOH, xe2x80x9calumina monohydratexe2x80x9d) which contains only one third of the amount of water and which is thermally stable until ca. 520xc2x0 C., i.e., until a temperature well above the useful range for effective flame retardancy of plastic materials.
Partial elimination of water as a means of stabilizing against subsequent early release of water vapour at elevated temperatures is accompanied by an increase in specifc surface area of the aluminium hydroxide. This increased surface is in the form of fine porosity which imparts hygroscopic properties to the surface of the thermally stabilized crystals. Increased specific surface area is further accompanied by higher oil absorption and higher viscosity in synthetic resins.
The problem to be solved by the present invention was to provide a simple and economic process for the production of aluminium hydroxides of improved thermal stability, high water content and low specific surface area.
According to the invention, this problem has been solved by the process of claim 1.
It has been found that aluminium trihydroxide can be partially dehydrated at elevated temperatures without a concomitant increase in the specific surface area and with little or no creation of aluminium oxide hydroxide, provided that the residence time at the dehydration temperature is sufficiently short. According to the present invention, this is accomplished by spray drying of an aqueous slurry of ordinary aluminium hydroxide at a drying gas temperature (at the inlet) of 400 to 600xc2x0 C.
Preferably, the drying gas temperature is 450 to 550xc2x0 C.
The exit gas temperature from the spray-dryer is preferably 200 to 300xc2x0 C.
The residence time of the aluminium hydroxide in the spray-dryer is preferably 0.5 to 10 s, more preferably 1 to 5 s.
Preferably, the average particle size of the starting aluminium hydroxide is 2 xcexcm or less. Advantageously, coarse particles (e.g.  greater than 10 xcexcm) are removed before spray drying. The aluminium hydroxide slurry used as starting material may be produced by crystallization from the sodium aluminate liquor of the Bayer process, filtering and thoroughly washing with hot water, and re-slurrying in an amount of water sufficient to obtain a slurry suitable for feeding to a spray-dryer. Typically, the solids content of the slurry is 40 to 60 wt. %.
The process of the present invention achieves the goal of improved thermal stability with no significant increase in the specific surface area of the aluminium hydroxide, as compared to the starting material, and without substantial decrease of the loss on ignition (LOI) values. Moreover, X-ray diffraction diagrams reveal the formation of trace quantities of aluminium oxide hydroxide (boehmite, AlOOH) only, hence indicating a maximum retention of the effective fire retardant aluminium trihydroxide.
The product of the present invention is of industrial importance especially in the application area of environment friendly printed circuit boards, where aluminium hydroxide can now be used to achieve the UL-94 V0 fire rating without the problems associated with the hygroscopic nature of the aluminium trihydroxide of the prior art, without raised viscosity levels and reduction in fire retardant effectiveness due to the presence of boehmite.
The following non-limiting examples illustrate the process of the present invention. The starting material in each example was aluminium hydroxide from different batches of the type Martinal(copyright) OL-104 (manufacturer: Alusuisse Martinswerk GmbH, Bergheim, Germany).